An airstrike hit the town center of Khan Sheikhoun, emitting a "poisonous gas," Anas al-Diab, an activist with the Aleppo Media Center (AMC) says.

Casualties are reported to have died from asphyxiation caused by exposure to an unknown gas or chemical agent. The injured are initially treated at a makeshift clinic near the scene and Al-Rahma hospital, among others.

Doctors Without Borders medics who visit the area's hospitals later say some victims appear to have symptoms consistent with exposure to a neurotoxic agent "such as Sarin gas or similar compounds."

Victims of a suspected chemical attack are seen in Khan Sheikhoun on Tuesday, April 4.

Three more strikes reported

Five minutes later, three additional strikes reportedly hit the town center, al-Diab says. This time there are no descriptions of gas.

At this point it is unclear if the planes involved are Syrian, but the rebel-held city is regularly targeted from the air by the Syrian military and its ally, Russia.

Russia later denies having warplanes in the vicinity.

Casualties arrive at hospital

Fares al-Jundi, a doctor who lives in a nearby village, hurries to the Maarat al-Numan hospital immediately after the airstrikes. "I've never seen anything like it, beyond description," he tells CNN as he describes the horrifying scenes.

A man carries a child at a makeshift hospital near the scene.

Al-Jundi estimates up to 500 people have been brought in; the wounded cover entire floors, from the patients' rooms and operating theaters and any remaining space in the corridors.

"I believe this horrible memory will stay with me for the rest of my life," he continues.

This photo, provided by the Idlib Media Center, shows several children who died in the targeted strikes.

Chaos on the ground

Hadi al Abdullah, an activist journalist, arrives on scene two hours after the initial attack.

Abdullah, who is suffering from blurred vision, a splitting headache and lethargy -- common side-effects of exposure to nerve gas -- says the scene is chaotic.

He tells CNN that that people were not initially aware of the presence of a toxic agent. Some who rushed to help are overwhelmed by the gas, including several members of the Syrian Civil Defense (SCD), or White Helmets, who have to be rescued by others from the group.

Abdullah says around 30 wounded people have gone to Turkey for treatment.

White Helmets transport victims away from the epicenter of the aerial bombardments.

Hospitals targeted in follow-up airstrikes, activists say

The makeshift clinic and Al-Rahma hospital, where the injured are being triaged, are targeted in new airstrikes four hours after the chemical attack, Idlib's health directorate and the SCD say in a joint press release.

Makeshift clinic near scene is devestated after follow-up airstrikes.

Videos emerge on social media

Videos, which appear to be from the scene, are posted quickly to social media platforms.

Mohammed Hardan, who works for AMC, says he was at al-Rahma hospital when it was targeted by aerial bombardment, posting footage of the incident hours later.

White Helmets: Residents targeted by poison gas

Warning: This section contains graphic images of dead children.

The White Helmets say at least 50 people, including women and children, have been killed by "poisonous gases."

70 dead, activists say

AMC activists say the Syrian regime is behind the attack and says at least 70 people are dead. It adds that 45 bodies have been identified so far.

The body of a baby is carefully wrapped by emergency responders.

Syrian army denies chemical attack

On Tuesday evening, Syrian President Bashar al-Assad's military "categorically denies" using chemical weapons and blames "terrorist" groups for the carnage.

But many in the international community blame the Assad regime for the outrage.

WEDNESDAY: Turkey says it has evidence of chemical attack

Recep Akdag, Turkey's Health Minister, tells Turkish media: ''We have findings that suggest there was a chemical attack. We are sending these findings to World Health Organization as well after documenting them. Humanity should not remain insensitive to this issue."

Turkey's Health Ministry later says 29 victims are being treated at six hospitals, and that three victims have died after their arrival.

55-year-old Aisha al-Tilawi recovers at a south Turkey hospital the day after the attack.

Survivors recall the horror

Survivors who made it to the Reyhanli Hospital on the Turkey-Syria border say they saw a plane drop chemical bombs.

Mazin Yusif, 13, tells CNN that he found his grandfather slumped over in front of his house shortly after the strike and ran for help. "I got dizzy and then fainted in front of my grandfathers garage. I next found myself here in this hospital, naked in a bed," he adds.

Mazin Yusif cries as he recalls the airstrike hitting his grandfather's house on Tuesday.

31-year-old Ahmed Abdel Rahim was also caught up in the strike. "I was in my house," he tells CNN. "I had difficulty breathing, but I feel better now. But I did throw up after getting to the hospital. I don't know if my family is dead or alive. I don't know anything."

Ahmed Abdel Rahim stares vacantly from his hospital bed as he describes the attack.

Syrians bury their dead on Wednesday. A father tells CNN he lost 25 members of his family, including his wife and 9-month-old twins. "My brothers, their children, and their cousins. Around 25 members of my family, all martyred," Abdul Hamid Youssef says, sobbing.

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Dan Kaszeta, a chemical weapons specialist, tells CNN the Russian version of events are "highly implausible," adding "it's much more plausible that Assad, who has used nerve agents in the past, is using them again."

The Russian Defense Ministry had earlier said on its Facebook page that the strike hit "workshops, which produced chemical warfare munitions."

A Syrian man collects samples in Khan Sheikhoun on Wednesday, April 5, 2017.

Trump: Attack is an 'affront to humanity'

US President Donald Trump says the airstrike has changed his views on Syria and Assad, calling it an "affront to humanity."

"It crossed a lot of lines for me. When you kill innocent children, innocent babies ... with a chemical gas that is so lethal that people were shocked to hear what gas it was, that crosses many, many lines -- beyond a red line," he says.

FRIDAY: US launches military strike

A satellite photograph shows the target of the US airstrike -- Al Shayrat airfield in Syria.

US warships launched 59 Tomahawk cruise missiles at Al Shayrat airfield, targeting aircraft, shelters and storage facilities. Six people were killed, according to a televised statement by the Syrian's Armed Forces General Command.

The strike is the first direct military action the US has taken against Assad's regime since the country's six-year civil war began, and could be interpreted by the Syrian government as an act of war.